Gaseous oxygen produced by air separation plants is usually at elevated pressure from about 20 to 50 bar. The basic distillation scheme is usually a double column process producing oxygen at the bottom of the low pressure column, operating at 1.4 to 4 bar. The oxygen must be compressed to higher pressure either by oxygen compressor or by the liquid pumped process. Because of the safety issues associated with the oxygen compressors, most recent oxygen plants are based on the liquid pumped process. In order to vaporize liquid oxygen at elevated pressure there is a need for an additional booster compressor to raise a portion of the feed air or nitrogen to higher pressure in the range of about 40 to 80 bar. In essence, the booster replaces the oxygen compressor. Pressurized air delivered by the booster compressor is condensed against the vaporizing liquid oxygen in a heat exchanger of the separation unit. This type of process is very power intensive and it is desirable to lower its power consumption when there exists another inexpensive supply of other forms of energy-latent streams, such as cryogenic liquid, pressurized gases, etc.
A typical liquid pumped process is illustrated in FIG. 1. In this type of process, atmospheric air is compressed by a Main Air Compressor (MAC) 1 to a pressure of about 6 bar absolute, it is then purified in an adsorber system 2 to remove impurities such as moisture and carbon dioxide that can freeze at cryogenic temperature to yield a purified feed air. A portion 3 of this purified feed air is then cooled to near its dew point in heat exchanger 30 and is introduced into a high pressure column 10 of a double column system in gaseous form for distillation. Nitrogen rich liquid 4 is extracted at the top of this high pressure column and a portion is sent to the top of the low pressure column 11 as a reflux stream. The oxygen-enriched liquid stream 5 at the bottom of the high pressure column is also sent to the low pressure column as feed. These liquids 4, 5 are subcooled before expansion against cold gases in subcoolers not shown in the figure for the sake of simplicity. An oxygen liquid 6 is extracted from the bottom of the low pressure column 11, pressurized by pump to a required pressure then vaporized in the exchanger 30 to form the gaseous oxygen product 7. Another portion 8 of the purified feed air is further compressed in a Booster Air Compressor (BAC) 20 to high pressure for condensation in the exchanger 30 against the vaporizing oxygen enriched stream. Depending upon the pressure of the oxygen rich product, the boosted air pressure can be around 65 bar or sometimes over 80 bar. The condensed boosted air 9 is also sent to the column system as feed for the distillation, for example to the high pressure column. Part of the liquid air may be removed from the high pressure column and sent to the low pressure column following subcooling and expansion. It is also possible to extract nitrogen rich liquid from the top of the high pressure column then pump it to high pressure (stream 13) and vaporize it in the exchanger in the same way as with oxygen liquid. A small portion of the feed air (stream 14) is further compressed and expanded into the column 11 to provide the refrigeration of the unit.
When a cryogenic liquid source is available at low cost, for example a liquid from a nearby air separation unit that produces liquid as a by-product, or a liquid produced by a liquefier that operates at night or during the time when power rates are low, or simply a low cost liquid from a surplus source, it is desirable to feed this liquid to the air separation plant to reduce its power consumption. However, when an air separation plant is fed with a liquid, some liquid products must be extracted from the plant by virtue of overall cold balance. However, since the liquid feed is already available at low cost, there is not much incentive to produce any significant amount of additional liquid products. Therefore, it is advantageous to provide a process capable of consuming those liquids efficiently.
The cold compression process as described in the prior art can be a good solution to the problem, since it uses the energy of refrigeration produced by the integrated expanders to yield efficient product compression.
A cold compression process, as described in U.S. Pat. No. 5,478,980, provides a technique to drive the oxygen plant with one single air compressor. In this process, air to be distilled is chilled in the main exchanger; then, further compressed by a booster compressor driven by a turbine exhausting into the high pressure column of a double column process. By doing so, the discharge pressure of the air compressor is in the range of 15 bar which is also quite advantageous for the purification unit. One inconvenience of this approach is the relatively high power consumption and an expander must be used to drive the process.
Some different versions of the cold compression process have also been described in U.S. Pat. Nos. 5,379,598, 5,901,576 and 6,626,008.
In U.S. Pat. No. 5,379,598, a fraction of feed air is further compressed by a booster compressor followed by a cold compressor to yield a pressurized stream needed for the vaporization of oxygen. This approach still has an expander as the main provider of refrigeration.
U.S. Pat. No. 5,901,576 describes several arrangements of cold compression schemes utilizing the expansion of vaporized rich liquid of the bottom of the high pressure column, or the expansion of high pressure nitrogen to drive the cold compressor. In some cases, motor driven cold compressors were also used.
U.S. Pat. No. 6,626,008 describes a heat pump cycle utilizing a cold compressor to improve the distillation process for the production of low purity oxygen for a double vaporizer oxygen process.